Field of the Invention
The present invention relates to an electric field sensor for underwater and, more particularly, to an electric field sensor for underwater that is installed on a seabed to detect a change in an electric field.
Description of the Related Art
A typical underwater object detection device is a SOund NAvigation and Ranging (SONAR).
As well known in the art, a SONAR senses noise (acoustic signal) emitted from a remote underwater sound source using a single or a plurality of underwater acoustic sensors and analyzes the acoustic signal, thereby determining characteristics and features of the aquatic or underwater sound source or locating the position of the aquatic or underwater sound source.
There are two types of underwater object detection methods: passive acoustic detection mode and active acoustic detection mode. More specifically, various technologies such as sonobuoy, SONAR, Towed Array Sonar System (TASS), Hull Mounted Sonar (HMS), and Hover Surveillance System (HSS) are used to detect a submarine. However, it was has been difficult to automatically detect a submarine over a wide range of water during long lengths of time.
An underwater object detection method using an acoustic signal is not an effective detection method in an area with heavy noise attributable to strong ocean current or an area with water layers being mixed due to a difference in salinity or temperature. Furthermore, it is difficult to detect an underwater object using an acoustic signal especially when a detection target is equipped with a sound-absorbing material.
There is an alternative method to detect an underwater moving object, whereby a moving object is detected by generating a magnetic field and detecting a change in the magnetic field attributable to the moving object. However, this technology has a disadvantage of having difficulty in detecting a moving object when there is another magnetic field that interferes with the magnetic field generated by the detection device.
Patent Document 1 discloses a technology to detect an underwater moving object using an electric field rather than an acoustic signal or a magnetic field.
The technology involves two direct current electrodes and a plurality of measurement electrodes to sense movement of a moving object having an electrical conductivity different from that of water. The technology detects a moving object by measuring a voltage between the measurement electrodes, the voltage being attributable to an electric field that is generated due to the voltage of the direct current electrodes.
As for the technology, sensitivity of a sensing member that senses an electric field is an important factor. The electrodes are conventionally made from graphite. Therefore, there are problems that it is difficult to manufacture a detection device employing the technology and the detection device made from graphite has low sensitivity.